or if u want to have quake style timedemo, hit ~ key, and type demoplay <urdemohere>?noframecap and it will be played as fast as possible. a word of warning though, demo play is no way same as in the game. your mileage may vary, but if you want to know how well your computer performance at UT, just turn on timedemo and play the game, at end of each map, there should be a block of msg regarding your game play, such as lowest and average FPS.
or if you feel fancy, bring down the console or hit tab key, and type stat fps, it should give all the info you need.
I downloaded a couple of demo's in the early days of UT, and they still work in the current version (4.20).
They are in-game demo's (style Quake3) with lots of action.
If you're interested, e-mail me at home (email@example.com) and I will mail you the zipfiles tomorrow.